The present invention relates to golf club shafts, and more particularly to golf club shafts having rifling on the interior surface thereof and sets of frequency matched golf club shafts where each shaft of the set has a predetermined stiffness distribution along its length directly related to the other shafts of the set.
U.S. Pat. No. 4,070,022, granted Jan. 24, 1978, describes a method of producing frequency matched golf club shafts, and the disclosure of this patent is incorporated herein by reference. Basically, the shafts of the frequency matched set are selected from a inventory of shafts classified as to frequency and length so that the frequencies of the individual shafts of the set fall on a predetermined gradient formed by a plot of shaft frequency and shaft length. Preferably the frequency gradient is a straight line that increases as shaft length decreases, and the frequency increments between successive shaft lengths along the gradient are substantial equal.
U.S. Pat. No. 4,122,593, granted Oct. 31, 1978, also describes a method of producing frequency matched golf club shafts and the disclosure of this patent is incorporated herein by reference. Individual golf club shafts are produced from a oversized universal blank of given length having a known natural frequency within a specified range. After the natural frequency of the blank is determined, selected amounts of shaft length are removed from both the tip and the butt end sections of the blank depending upon (1) the natural frequency of the blank and (2) the desired frequency of the finished golf club shaft. The total amount removed from the blank solely depends upon the length of the shaft being produced. On the other hand, the selected proportional amounts removed from the tip and butt end sections of the blank solely depend upon the desired natural frequency of the shaft. When increased amounts of shaft length are removed from the butt end section the shaft being produced becomes more flexible. Conversely, shafts produced by removing greater amounts from the tip end section produce stiffer shafts.
For example, following the process explained in U.S. Pat. No. 4,122,593, a golf club shaft having a natural frequency of 310 cycles per minute may be produced from oversized universal blank A having a natural frequency of 282 cycles per minute by removing selected amounts from the tip and butt end sections of the blank. A shaft having the same natural frequency of 310 cycles per minute may also be produced from universal blank B having a natural frequency of 262 cycles per minute. However, in order to achieve the target frequency of 310 cycles per minute less length is removed from the butt end section of blank B and more length is removed from the tip end section. In other words, golf club shafts having a target natural frequency and a given length may be produced from a number of different universal blanks each having the same length but a different natural frequency. In each case the total amount of length removed is the same but the proportional amounts removed from the tip and butt end sections are different. While this procedure is useful for producing frequency matched sets of golf club shafts where each shaft in the set has a predetermined frequency that falls on a gradient, the stiffness distribution along the length of each shaft of the set is different and unrelated to the others because the frequency of the universal blanks used to produce the set are different.
Tubular metal golf club shafts are traditionally formed by tube milling techniques well known in the art. Essentially flat metal stock is formed into a tube the seam of which is secured by welding. The formed tube is then pulled over a series of plug mandrels until the desired cross-sectional configuration of the tube is obtained. Once the wall thickness and diameter of the tube are within a desired range the tube is cut into unit lengths, and golf club shafts are produced from these lengths by swaging techniques well known in the art. Wall thickness must be sufficient to impart the necessary strength and stiffness to the golf club shafts but excess wall thickness is avoided because it adversely contributes to the weight of the shaft.